Pong Ball loses momentum

I'm currently making a pong game, but everytime the ball hits the paddles or walls, it loses momentum and slows down.

using UnityEngine; using System.Collections; using UnityEngine.UI; // needed for Text

public class ballscript : MonoBehaviour {

 public float ballInitialVelocity = 600f;
 public Text myText;

 private Rigidbody rb;
 private bool ballInPlay=false;

 void Awake()
 {
     rb = GetComponent<Rigidbody>();

 }



 void Update()
 {
     if (Input.GetButtonDown("Fire1") && ballInPlay == false)
     {

         ballInPlay = true;
         rb.isKinematic = false;
         rb.AddForce(new Vector3(ballInitialVelocity, ballInitialVelocity, 0));
     }

     if (transform.position.x < -40 || transform.position.x > 20)
     {
         ballInPlay = false;
         Vector3 playerPos = new Vector3(0, -0.0f, -0.5f);
         transform.position = playerPos;
         rb.velocity = Vector3.zero;
     }
 }

}